A conventional spark plug is used for an ignition of internal-combustion engines, such as an automotive engine. The spark plug includes a center electrode, an insulator accommodating the center electrode in its axial bore, a metal shell surrounding and holding a radial circumference of the insulator, and a ground electrode having one end thereof that is joined to the metal shell and the other end thereof that forms a spark discharge gap with the center electrode. A spark discharge is conducted between the center electrode and the ground electrode to thereby ignite an air-fuel mixture.
Recently, automotive engines have been advanced with respect to high power and fuel efficiency, and a reduction in the size of a spark plug has been demanded in order to maintain freedom of design. In connection with this, a clearance between an outer circumferential face of the insulator and an inner circumferential face of the metal shell is reduced, whereby side sparks are caused by a lower potential difference than in a conventional case. In particular, an electric field tends to concentrate on an annular edge formed by a front end face and an inner circumferential face of the metal shell. Thus, a spark plug having miniaturized parts of the conventional spark plug is likely to cause side sparks in a region from the outer circumferential face of the insulator to the edge when carbon fouling occurs. In such a case, concentration of the electric field can be ameliorated if the annular edge formed by the front end face and the inner circumferential face of the metal shell is chamfered. As a result, side sparks are prevented. For example, see Japanese Patent Application Laid-Open (kokai) No. 2003-68420, hereinafter referred to as Patent Document 1.
However, when the spark plug is miniaturized, the metal shell is also reduced in thickness. When a certain area of the front end face of the metal shell, which effectively ameliorates the concentration of the electric field, is chamfered, a ratio of a flat area remaining in the front end face is smaller than that of the conventional spark plug. If an end face of the ground electrode is joined to this small flat area in the front end face, a contact area of the metal shell and the ground electrode is small whereby sufficient joint strength is unlikely to be obtained. Thus, it has been disclosed that a chamfered area of the front end face of the metal shell is further increased compared to the conventional spark plug so that an inclined face (joint face) formed by chamfering is wider. Then, the entire end face (joint end face) of the ground electrode (creeping face ground electrode) is brought into contact with the inclined face and joined thereto while the contact area therebetween is secured. For example, see Japanese Patent Application Laid-Open (kokai) No. 2005-50746, hereinafter referred to as Patent Document 2.
A normal ground electrode is formed in such a manner that a rod-like base material having a rectangular cross section is cut into a rectangular parallelepiped shape so that a sectioned face of the base material is perpendicular to an extending direction of the base material. Thus, when the ground electrode is simply joined to the inclined portion of the front end face of the metal shell, the front end side of the ground electrode inwardly extends and comes close to the front end portion of an insulator as a creeping ground electrode of Patent Document 2. As a result, side sparks tend to occur when carbon fouling arises. In such a case, as an aerial electrode disclosed in Patent Document 2, an end face of the electrode is preferably made into a slant face with respect to an extending direction so that the ground electrode extends in an axial direction of a spark plug when joined to the metal shell.
However, when bending a ground electrode to form a spark discharge gap, a portion of the ground electrode close to a joint face to the metal shell tends to start bending inwardly because the ground electrode is pressed against a mold so that an outer face thereof fits in the mold. Then, as in the above-mentioned creeping ground electrode, a portion from a bending part to the front end portion of the ground electrode inwardly extends in a slanted manner, whereby the flame kernel growing in the spark discharge gap is likely to be in contact with the ground electrode. As a result, it is likely to cause deterioration in ignitability. Further, since an inner face of the ground electrode comes close to a front end portion of the insulator, spark discharge tends to occur in a portion other than the spark discharge gap (i.e., side sparks) when a spark plug is fouled. In order to effectively prevent side sparks, a sufficiently sized gap (a clearance) surrounded by a face of the ground electrode facing the center electrode, the center electrode and the insulator is necessary.
The present invention is accomplished in order to solve the above-mentioned problems, and an object of the present invention is to provide a spark plug capable of securely preventing side sparks which are caused between the ground electrode, the center electrode and the insulator when the spark plug is fouled, and to provide a method for manufacturing the spark plug.